This invention relates to a heatsealable laminated film and to the method for production of the film.
It is well known to produce a heatsealable film by laminating polypropylene film with an ionomer as described in U.S. Pat. No. 3,355,319 and Japanese Patent Publication No. 46-40793.
The films described in U.S. Pat. No. 3,355,319 are film laminates made by extruding an ionomer resin onto one surface of a biaxially oriented polypropylene film as shown in Example XII of the patent. However, this film has the following disadvantages:
(1) This film exhibits weak adhesive strength between the polypropylene base layer and the ionomer layer with the result that a weak heatseal is formed when the base layer is homopolypropylene and the film is made by laminating a biaxially oriented polypropylene film with an ionomer resin without stretching.
(2) When the film is made by laminating one side of the base layer with ionomer, it curls easily.
The method described in Japanese Patent Publication No. 46-40793 relates to a process for the production of heatsealable film laminates comprising laminating a uniaxially oriented polypropylene film with an ionomer, and thereafter stretching the laminated film in the transverse direction. The adhesive strength between two layers of this film is relatively improved by transverse stretching after lamination in comparison with that of the film described in U.S. Pat. No. 3,355,319, but this film also has the following disadvantages:
(1) This film exhibits thickness variations or surface roughness because the stretching ratio cannot increase, and the ionomer layer sticks to the chill roll when the laminated layer is thin.
(2) When this film is made by laminating one side of the base layer with an ionomer, it curls easily because there is an orientation difference between the two layers. When this film curls, it cannot be fed into automatic packaging machines and cannot practically be used for automatic packaging applications.
(3) When the ionomer is laminated on a uniaxially oriented polypropylene film, delamination between the ionomer layer and polypropylene layer often takes place on the nip roll so that the ionomer easily adheres to the roll. Thus, it is difficult to commercially produce this type of film because of the delamination problem.
Accordingly, it is an object of this invention to remove these disadvantages without sacrificing good heat sealing properties, transparency, and scratch resistance.
The present invention provides a laminated polypropylene film comprising a base layer of polypropyleneionomer blend coated with ionomer layer(s), wherein the blend consists of 92 to 99 wt. % polypropylene and 1 to 8 wt. % ionomer. By blending 1 to 8 wt. % ionomer into the base layer, most of aforementioned disadvantages are removed and, hence, good laminated film can be made.
This invention also relates to a method for the preparation of film laminates by a coextrusion process. It should be pointed out that it is difficult to co-extrude film laminates comprising a base layer of homopolypropylene coated with ionomer layer(s), as described in U.S. Pat. No. 3,355,319 and Japanese Patent Publication No. 46-40793 for the following reason:
When the coextruded laminate is stretched in the longitudinal direction, the ionomer layer sticks to the preheating rolls because the melting point of the ionomer is lower than the temperature at which polypropylene film is longitudinally stretched. Therefore, it is very difficult to make laminated film by the conventional stretching process.
This sticking tendency becomes worse when the film is commercially produced at high speed. If the longitudinal stretching temperature is lowered in order to avoid sticking between the film and rolls, the film has a very high molecular orientation and, hence, tends to split along lines parallel to the stretching direction, which leads to a film break when it is transversely stretched. If the film is stretched under the same conditions without very high orientation, it cannot be commercially used for packaging applications because of poor mechanical properties such as low tensile strength, low Young's modulus, and large elongation, etc.
Another method to avoid sticking is to use nonsticky rolls, such as fluoro-resin covered rolls and special rubber-covered rolls, in the longitudinal stretching process, but these rolls are not satisfactory for the present purpose.
In the present invention, the orientation of the film laminae is lowered by blending 1 to 8 wt. % ionomer into the loose layer. Accordingly, film laminae, having suitable orientations, can be obtained by lower stretching temperatures. Therefore, the film laminae can be produced by a coextrusion process without adhesion to the roll.
Heretofore, when polypropylene and an ionomer have been co-extruded, the boundary layer or layers between the two resins exhibit a propensity to become rough (hereinafter, this phenomanon is referred to as "boundary roughening"). Therefore, the biaxially oriented film has poor transparency and gloss. This boundary roughening is considered to be caused by the difference of viscosity and shear rate at the boundaries, and by the difference of volume shrinkage when molten polymers become solid. This roughening tendency becomes worse with the decreasing thickness of the co-extruded ionomer layer or layers.
Prior art biaxially oriented polypropylene films laminated with an ionomer layer have a propensity to curl. This propensity causes trouble in packaging machines or bagmaking machines. A desirable curl ratio is less than 10 percent, preferably less than 5 percent.
In addition to the aforementioned disadvantages, the laminated films made in accordance with the prior art have other disadvantages, such as poor gauge control and poor adhesion strength between the base layer and laminated layer or layers.
The present invention presents a distinct improvement over the known prior art in the above-mentioned characteristics.
In the present invention, boundary roughening is improved with the results of good gloss and transparency, because the incorporation of an ionomer into the base layer reduces the differences of melt viscosity, shear rate and volume shrinkage between the base and laminated layer or layers. The ionomer can be regarded as a polymer compatible with polypropylene if the amount of ionomer is limited.
The incorporation of ionomer into the base layer inhibits the formation of very high molecular orientation in the longitudinal stretching zone. In other words, the optimum stretching temperature can be lowered by about 5-15 deg. C by the incorporation of an ionomer. Therefore, the film of this invention can be easily stretched, without sticking, on stretching rolls covered with fluoro-resin or special rubbers.
The cast film in accordance with this invention has a highly uniform surface due to the good contact between the casting drum surface and the ionomer layer, resulting in the improvements of neck-down, film surface roughness, stretchability, gauge control and curling propensity.
Moreover, the adhesive strength between both layers in the cast film is improved by blending an ionomer into the polypropylene base layer.
Further, a film made from a polypropyleneionomer blend has been proposed by British Pat. No. 995,802. However, this invention relates to films having improved dyeability. Further, the disclosed film exhibits poor heatsealability since it is mono-layered film, not laminated film.